1. Field of the Invention
The present invention relates to a thin film transistor device and a liquid crystal display apparatus and, more specifically, to a high-performance thin film transistor device that allows its threshold value to be adjusted and a liquid crystal display apparatus that offers high-quality functions and an excellent image quality.
2. Description of the Related Art
Conventional active matrix liquid crystal display apparatuses comprise thin film transistors (TFT) arranged in a matrix on a glass substrate corresponding to pixels. The liquid crystal display apparatus drives the thin film transistors by providing sequentially them with image data through a separately manufactured driving circuit and then takes advantage of the orientation of liquid crystals to present an image.
Thus, the conventional liquid crystal display apparatus needs manufacturing steps in which the external driving circuit is mounted and a TFT board is connected to the driving circuit, and such steps are one of the factors that impedes a cost reduction of the liquid crystal display apparatus.
TFTs are typically manufactured by forming semiconductor thin film layers of amorphous silicon or polysilicon on a glass substrate. Although arranging a driving circuit on the substrate in a TFT structure is contemplated, power consumption arising from through currents is increased because a leakage current in the TFT structure is large compared with a driving circuit based on a crystalline silicon. For example, to construct a D/A converter, a number of TFTs will be required, and the amount of leakage current is accordingly increased.
When a TFT is employed as a switching device for a pixel, the leakage current in the TFT greatly affects image quality. Specifically, the TFT leakage current lowers a voltage between liquid crystals, degrading an image contrast. This also makes a multi-level gradation image presentation difficult.
Since a high-density and large screen liquid crystal display is in growing demand, amorphous silicon that permits a large area, uniform film forming at low temperatures is preferred. However, one of the disadvantages of amorphous silicon is its large leakage current, which presents difficulty presenting a high-contrast, multi-level gradation image. In view of a large mobility in amorphous silicon, a device needs to be enlarged, and the leakage current is even more increased.